EXPLANATION OF THE PLATES.
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PLATE IV.
Fig. 1. is a section of the parts of a high pressure engine with a four-passaged
cock. The engine is supposed to be partly within the boiler, of which D B is the
top plate. P is the steam piston, and R the piston rod, A is the four-passaged
cock; the steam enters from the boiler at S, and passes through t to the top of the
piston, and the steam below escapes through the passage b, and pipe a and E, to
the atmosphere; the pipe E is surrounded by water, which the escaping steam
warms ready for the boiler. By turning the cock the motions are reversed, but it
is obvious we cannot in this engine employ the expanding force of the steam. The
motion is regulated by a throttle valve V. See art. 356—361.
Fig. 2. and 3. show a section and plan of a similar engine, with a D-slide
instead of a cock ; the steam enters from the boiler at S, and by the passages being
shut and opened close to the extremities of the cylinder, there is no loss by the
communicating pipes being filled with strong steam. See art. 364. This engine
will not work expansively unless the construction of the slide be altered. See
art. 371. Contrary to the usual practice, the packing of the slide is on the sliding
part; the advantage of this plan is obvious, but the practical difficulty of boring
a semi-cylinder is incurred.
Fig. 4. is a simple arrangement of the high pressure engine by which the
expanding power of the steam may be used ; it is the invention of Murdoch. The
passages are opened and closed by pistons sliding in a pipe : the steam enters this
pipe at S, and the steam is supposed to be just shut off by the upper piston, so that
by the expansion of that in the cylinder the rest of the stroke is completed, the
passage E to the atmosphere being still open. See art. 371—380. The slide
would be improved by making it of the form of a D-slide.
The construction of the pistons of the slide is a suggestion which may perhaps
answer better than the common ones, (art. 450. and note.)
Fig. 5. is an arrangement to illustrate the action of a high pressure engine to
work expansively by means of combined cylinders. See art. 381—383.